1. Field of the Invention
The present invention relates to an image forming process which is capable of speedily forming images of higher qualities.
2. Related Background Art
The ink-jet ejecting method that facilitates the acceleration of the recording speed, enhances resolution, lowers noise, diversifies image colors and broadens adaptability to diversified recording patterns is rapidly prevailing in recent days for recording letters, such as Chinese characters first of all, obtaining hard copies of various kinds of figure information, and for other purposes of use. Owing to compatibilities with various kinds of inks and the adaptability to diversified recording patterns, the ink-jet ejecting method is attracting attention as means for obtaining color hard copies of the output of computer terminals and so on. Further, attempts are being made to apply a multi-color ink-jet ejecting method even to the fields of multi-color printing and color- photograph printing since the multicolor ink-jet ejecting method is capable of providing images that are not inferior in quality to those formed by the ordinary multi-color printing and allows printing of a small number of copies at a cost lower than that required for the ordinary plate-making method. Though aqueous dyes have conventionally been used exclusively as color materials of inks for the ink-jet ejecting method, pigments are now usable as color materials for inks, thereby making it possible to obtain a clear image having excellent color tones.
The ink-jet ejecting method is generally classified into: a pressure oscillation type, such as an electric-charge control method and an electric-field control method; a static-electricity acceleration type that electrostatically accelerates ink with a voltage applied across electrodes opposed to each other; a pressure-pulse type, such as an on-demand type that pushes out ink with pressure pulses; and an ink-mist type that produces an ink mist with an ultrasonic oscillation; each of which forms an image by attaching drops of a liquid ink to a recording medium.
However, paper of certain high grades, coated papers, baryta papers and resin-coated papers used as substrates of photographic printing paper, transparent polymer films used as light transmissive recording media for overhead projectors, etc. are inferior in ink absorptivity, whereby unabsorbed ink may remain for a long time on the surfaces of recording media when these papers are used as recording media for the ink-jet ejecting method. In such a case, if the unabsorbed ink is brought into contact with portions of printers, operators touch the unabsorbed ink or sheets, which are discharged successively and overlap so as to rub recording surfaces, images may be fouled or the inks may flow out, thereby making it impossible to obtain clear images.
In order to record images of high quality on such recording media by the ink-jet ejecting method, it is conceivable to dispose an ink-receiving layer that has an ink absorptivity high enough to speedily absorb an ink attached to a recording medium, thereby setting it in an apparently dry condition. An ink-receiving layer that has a large number of voids can be mentioned as an example of an ink- receiving layer having an ink absorptivity high enough to meet this requirement.
In order to form an ink receiving layer which has a high ink absorptivity, it is carried out, for example, to use thermoplastic resin particles as a material of an ink receiving layer which absorbs and holds an ink into voids formed among the particles. When the voids formed among the particles have a size smaller than a color material contained in the ink, however, the color material may remain on a recording medium, thereby posing a problem in fretting resistance of a formed image. An image formed with an ink which uses a dye as a color material poses no serious problem in the fretting resistance or water resistance. This fact is considered due to a fact that particles of the dye dissolved in the ink are sufficiently small and can easily penetrate voids in an ink receiving layer of a recording medium. In case of an image formed with an ink which contains a pigment larger than the dye, in contrast, its fretting resistance and water resistance are influenced by a size of the pigment and a size of the voids. In other words, the ink-jet ejecting method which uses an ink containing a pigment allows a quality of an image to be influenced by matching between an ink and a recording medium, or is hardly capable of stably forming images of high qualities.
The present invention has been achieved in view of the aforementioned background and an object of the present invention is to provide an image forming process which makes it possible to stably obtain an image of higher quality by utilizing an ink-jet ejecting method which uses an ink containing a pigment.
According to one aspect of the present invention, there is provided an image forming process comprising steps of:
(i) providing a recording medium having a substrate and a thermoplastic resin layer which constitutes an outermost layer of the recording medium;
(ii) ejecting an aqueous ink containing a pigment which can be dispersed into water without a dispersing agent toward an outside surface of the thermoplastic resin layer by an ink-jet ejecting method, thereby attaching the pigment to the outside surface of the thermoplastic resin layer; and
(iii) transferring the pigment from the outside surface of the thermoplastic resin layer into the thermoplastic resin layer.
The inventors made various examinations to accomplish the object described above and found this fact: a pigment may not transfer smoothly into a thermoplastic resin layer when a general pigment ink which contains a pigment and a dispersing agent therefor is attached to a recording medium having a surface composed of a thermoplastic resin layer, and then the recording medium is heated, whereas an image of an extremely high quality can be formed by similar procedures when carbon black as a pigment which can be dispersed into water without a dispersing agent (hereinafter referred to in short as a self-dispersion type carbon black) is selected as a color material and an ink to which no dispersing agent is added is used. On the basis of this knowledge, the inventors considered that the dispersing agent contained in the general pigment ink governs the mutual solubility between a pigment and a thermoplastic resin and the mobility of a pigment on a surface of a thermoplastic resin layer into the thermoplastic resin layer, thereby achieving the present invention.
The image forming process according to the embodiment of the present invention allows a pigment, which is attached to a surface of a thermoplastic resin layer by an ink-jet ejecting method, to transfer into the thermoplastic resin layer so that it is covered with the thermoplastic resin, thereby being capable of forming an image of an extremely high quality, which is excellent in fretting resistance, water resistance and durability. Further, the image forming process according to the present invention achieves a quality of an image which being not influenced by the relationship between the size of a pigment and the size of pores in a surface of a recording medium, thereby making it possible to form images of a uniform quality.